Zoltán Kolláth

Zoltán Kolláth
Eötvös Loránd University · Savaria Department of Physics

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149
Publications
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Publications

Publications (149)
Article
Full-text available
The spatial and angular emission patterns of artificial and natural light emitted, scattered, and reflected from the Earth at night are far more complex than those for scattered and reflected solar radiation during daytime. In this commentary, we use examples to show that there is additional information contained in the angular distribution of emit...
Article
In recent decades, considerable research has been carried out both in measuring and modelling the brightness of the sky. Modelling is highly complex, as the properties of light emission (spatial and spectral distribution) are generally unknown, and the physical state of the atmosphere cannot be determined independently. The existing radiation trans...
Article
Interest in investigation of the impact of light pollution on ecology leads to a surge of observations and measurements. This article presents the measurement of the night sky radiance in over a period of 4.5 years in the city of Bremen, Germany from two sites situated in suburban and rural areas. It was found that both sites show a decreasing tren...
Article
We have started a light pollution survey in the Hungarian national parks. We use a mobile laboratory consisting of digital cameras and robotic panorama heads to gather high-resolution panorama imaging radiometry. The same type of cameras with fish-eye lenses are used in fix monitoring stations. Parallel to the imaging radiometry, we measure the spe...
Article
Full-text available
The natural light conditions and the artificial lighting of the habitats affect the quality of bird territories and the territorial behaviour of birds. However, we know almost nothing about the evolutionary consequences of light on birds. In the biomonitoring part of the project entitled “ Development of international research environment for light...
Article
Full-text available
The measurement of night sky quality has become an important task in nature conservation. The primary device used for this task can be a calibrated digital camera. In addition, colour information can be derived from sky photography. In this paper, we provide a test on a concept to gather information about the possible sources of night sky brightnes...
Article
Full-text available
The measurement of night sky quality has become an important task in night sky conservation. Modern measurement techniques involve mainly a calibrated digital camera or a spectroradiometer. However, panchromatic devices are still prevalent to this day, even in the absence of determining the spectral information of the night sky. In the case of mult...
Article
The International Dark Sky Association has a flight of rules for public lighting in dark sky parks. In Hungary, many settlements still illuminate with compact fluorescent lights and sodium lamps, that do not fulfill these rules. In our research project, financed by a grant of the European Union, we established a system of Living Environmental Labor...
Article
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Atmospheric conditions can significantly affect the sky brightness originating from artificial lights. Previous works studied the cloudiness, cloud base height, optical depth of cloud, aerosol optical depth and aerosol scale height as atmospheric parameters affecting night sky brightness. Instead of using these parameters as a simplification of the...
Article
Full-text available
Multi-spectral imaging radiometry of the night sky provides essential information on light pollution (skyglow) and sky quality. However, due to the different spectral sensitivity of the devices used for light pollution measurement, the comparison of different surveys is not always trivial. In addition to the differences between measurement approach...
Article
Pulsations of RV Tauri-type variable stars can be governed by chaotic dynamics. However, observational evidence for this happening is usually hard to come by. Here we use the continuous, 4-yr-long observations of the Kepler space telescope to search for the signs of chaos in the RVb-type pulsating supergiant, DF Cygni.We use the Global Flow Reconst...
Preprint
Pulsations of RV Tauri-type variable stars can be governed by chaotic dynamics. However, observational evidence for this happening is usually hard to come by. Here we use the continuous, 4-year-long observations of the Kepler space telescope to search for the signs of chaos in the RVb-type pulsating supergiant, DF Cygni. We use the Global Flow Reco...
Article
Full-text available
We present and discuss the pulsational characteristics of the Delta Scuti star 38 Eri from photometric data obtained at two widely spaced epochs, partly from the ground (1998) and partly from space (MOST, 2011). We found 18 frequencies resolving the discrepancy among the previously published frequencies. Some of the frequencies appeared with differ...
Article
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Artificial skyglow is dynamic due to changing atmospheric conditions and the switching on and off of artificial lights throughout the night. Street lights as well as the ornamental illumination of historical sites and buildings are sometimes switched off at a certain time to save energy. Ornamental lights in particular are often directed upwards, a...
Conference Paper
Full-text available
For mapping and monitoring the changes in light pollution one of the convenient ways is to measure the radiances from space. VIIRS sensor onboard the Suomi NPP satellite is capable to measure visual radiances (VIIRS Day/Night band in 0.5-0.9 micrometre wavelength range). Our attention is to detect changes in the satellite based radiances and mappin...
Article
Full-text available
Measuring the brightness of the night sky has become an increasingly important topic in recent years, as artificial lights and their scattering by the Earth’s atmosphere continue spreading around the globe. Several instruments and techniques have been developed for this task. We give an overview of these, and discuss their strengths and limitations...
Article
Full-text available
Artificial skyglow is constantly growing on a global scale, with potential ecological consequences ranging up to affecting biodiversity. To understand these consequences, worldwide mapping of skyglow for all weather conditions is urgently required. In particular, the amplification of skyglow by clouds needs to be studied, as clouds can extend the r...
Article
National parks and protected areas have an important role in protecting starry sky and the undisturbed nighttime environment. Hortobágy which is one of the darkest areas in Hungary, became an International Dark Sky Parks recently. Its significance is mostly related to the protection of the high biodiversity which is endangered by the effects of li...
Article
Full-text available
Near all-sky imaging photometry was performed from a boat on the Gulf of Aqaba to measure the night sky brightness in a coastal environment. The boat was not anchored, and therefore drifted and rocked. The camera was mounted on a tripod without any inertia/motion stabilization. A commercial digital single lens reflex (DSLR) camera and fisheye lens...
Article
Full-text available
A crucial part of the qualification of international dark sky places (IDSPs) is the objective measurement of night time sky luminance or radiance. Modern digital cameras provide an alternative way to perform all sky imaging either by a fisheye lens or by a mosaic image taken by a wide angle lens. Here we present a method for processing raw camera i...
Article
We study the different patterns of interannual magnetic variability in stars on or near the lower main sequence, approximately solar-type (G-K dwarf) stars in time series of 36 years from the Mount Wilson Observatory Ca\,{\sc ii}\,H\&K survey. Our main aim is to search for correlations between cycles, activity measures and ages. Time-frequency anal...
Article
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We report on luminance measurements of the summer night sky at a field site on a freshwater lake in northeastern Germany (Lake Stechlin) to evaluate the amount of artificial skyglow from nearby and distant towns in the context of a planned study on light pollution. The site is located about 70 km north of Berlin in a rural area possibly belonging t...
Article
The public lighting system has been remodelled in several Hungarian cities. In some cases the majority of the old luminaries were fitted with high pressure sodium lamps and they were replaced with white LED lighting with a typical correlated colour temperature of about 4500 K. Therefore, these remodelling works provide a testbed for methods in meas...
Article
Hydrodynamic models of RR Lyrae pulsation display a very rich behaviour. Contrary to earlier expectations, high order resonances play a crucial role in the nonlinear dynamics representing the interacting modes. Chaotic attractors can be found at different time scales: both in the pulsation itself and in the amplitude equations shaping the possible...
Article
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A detailed analysis is presented of 33 RR Lyrae stars in Pisces observed with the Kepler space telescope over the 8.9-d long K2 Two-Wheel Concept Engineering Test. The sample includes not only fundamental-mode and first-overtone (RRab and RRc) stars but the first two double-mode (RRd) stars that Kepler detected and the only modulated first-overtone...
Article
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We performed a non-linear dynamical analysis on the Blazhko modulation for the first time. Our results suggest that the detection of chaotic nature behind the phenomenon is limited by the instrumental effects and the data processing problems of the Kepler pipeline concerning high-amplitude variable stars.
Article
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We present a detailed non-linear dynamical investigation of the Blazhko modulation of the Kepler RR Lyrae star V783 Cyg (KIC 5559631). We used different techniques to produce modulation curves, including the determination of amplitude maxima, the O−C diagram and the analytical function method. We were able to fit the modulation curves with chaotic...
Article
Full-text available
In order to benefit from the four year unprecedented precision of the Kepler data, we extracted light curves from the pixel photometric data of the Kepler space telescope for 15 Blazhko RR Lyrae stars. To collect all the flux from a given target as accurately as possible, we defined tailor-made apertures for each star and quarter. In some cases, th...
Article
Multi-wavelength imaging luminance photometry of sky glow provides a huge amount of information on light pollution. However, the understanding of the measured data involves the combination of different processes and data of radiation transfer, atmospheric physics and atmospheric constitution. State of the art numerical radiation transfer models pro...
Article
We characterized the time intervals between the interchanges of the alternating high- and low-amplitude extrema of three RV Tauri and three RR Lyrae stars.
Article
We present and discuss an extensive data set for the non-Blazhko ab-type RR Lyrae star SDSSJ015450+001501, including optical SDSS ugriz light curves and spectroscopic data, LINEAR and CSS unfiltered optical light curves, and infrared 2MASS JHKs and WISE W1 and W2 light curves. Most notably, light curves obtained by 2MASS include close to 9000 photo...
Article
We performed a comparative dynamical investigation of chaotic test data using the global flow reconstruction method. We demonstrate that large-amplitude, long-term variations may have a disturbing effect for the analysis. The Empirical Mode Decomposition method (EMD) and the Fourier filtering were tested to remove the additional variations. Test re...
Article
We examined the complete short cadence sample of Kepler RR Lyrae stars to further investigate the recently discovered dynamical effects such as period doubling and additional modes. Here we present the findings on four stars. V450 Lyr may be a non-classical double-mode RR Lyrae star pulsating in the fundamental mode and the second overtone. In thre...
Article
The recent precise photometric observations and successes of the modelling efforts transformed our picture of the pulsation of RR Lyrae stars. The discovery of additional frequencies and the period doubling phenomenon revealed that a significant interaction may occur between pulsational modes. The signs of irregularities were detected both in obser...
Article
Recent theoretical and observational findings breathed new life into the field of RR Lyrae stars. The ever more precise and complete measurements of the space asteroseismology missions revealed new details, such as the period doubling and the presence of the additional modes in the stars. Theoretical work also flourished: period doubling was explai...
Article
The observations of the Kepler space telescope revealed that fundamental-mode RR Lyrae stars may show various radial overtones. The presence of multiple radial modes may allow us to conduct nonlinear asteroseismology: comparison of mode amplitudes and frequency shifts between observations and models. Here we report the detection of three radial mod...
Article
Solar variability and its effects on the physical variability of our (space) environment produces complex signals. In the indicators of solar activity at least four independent cyclic components can be identified, all of them with temporal variations in their timescales. Time-frequency distributions (see Kolláth & Oláh 2009) are perfect tools to d...
Article
The Blazhko phenomenon, the modulation of the pulsation of RR Lyrae stars, remains one of the most stubborn unsolved problems of stellar pulsation. The recent idea of Stothers proposes that periodic variations in the properties of the convective envelope may be behind the amplitude and phase modulation. In this work, we approximated the mechanism b...
Article
The period doubling phenomenon was recently discovered in RR Lyrae stars with the Kepler space telescope and has been theoretically explained by hydrodynamic calculations. However, peculiar solutions of the Florida-Budapest turbulent convective hydrodynamic code suggest that bifurcation cascade may evolve to chaos in these dynamical systems. We sho...
Article
Full-text available
The Kepler space telescope revealed new, unexpected phenomena in RR Lyrae stars: period doubling and the possible presence of additional modes. Identifying these modes is complicated because they blend in the rich features of the Fourier-spectrum. Our hydrodynamic calculations uncovered that a 'hidden' mode, the 9th overtone is involved in the peri...
Article
Full-text available
The origin of the conspicuous amplitude and phase modulation of the RR Lyrae pulsation - known as the Blazhko effect - is still a mystery after more than 100 years of its discovery. With the help of the Kepler space telescope we have revealed a new and unexpected phenomenon: period doubling in RR Lyr - the eponym and prototype of its class - as wel...
Article
Full-text available
We report results of initial work done on selected candidate Cepheids to be observed with the Kepler space telescope. Prior to the launch, 40 candidates were selected from previous surveys and data bases. The analysis of the first 322 d of Kepler photometry, and recent ground-based follow-up multicolour photometry and spectroscopy allowed us to con...
Article
We investigated period doubling, a well-known phenomenon in dynamical systems, for the first time in RR Lyrae models. These studies provide theoretical background for the recent discovery of period doubling in some Blazhko RR Lyrae stars with the Kepler space telescope. Since period doubling has been observed only in Blazhko-modulated stars so far,...
Article
The Blazhko effect is a long term, generally irregular modulation of the light curves that occurs in a sizeable number of RR Lyrae stars. The physical origin of the effect has been a puzzle ever since its discovery over a hundred years ago. We build here upon the recent observational and theoretical work of Szabo et al. on RRab stars who found with...
Article
We present our analysis of Kepler observations of 29 RR Lyrae stars, based on 138-d of observation. We report precise pulsation periods for all stars. Nine of these stars had incorrect or unknown periods in the literature. Fourteen of the stars exhibit both amplitude and phase Blazhko modulations, with Blazhko periods ranging from 27.7 to more than...
Article
The first detection of the period doubling phenomenon is reported in the Kepler RR Lyrae stars RR Lyr, V808 Cyg and V355 Lyr. Interestingly, all these pulsating stars show Blazhko modulation. The period doubling manifests itself as alternating maxima and minima of the pulsational cycles in the light curve, as well as through the appearance of half-...
Article
Irregular behaviour of pulsating variable stars may occur due to low dimensional chaos. To determine the quantitative properties of the dynamics in such systems, we apply a suitable time series analysis, the global flow reconstruction method. The robustness of the reconstruction can be tested through the resultant quantities, like Lyapunov dimensio...
Article
Detailed analysis of free parameters of the Florida-Budapest turbulent convective stellar model were carried out. A recent explanation of the Blazhko effect ties the observed modulation to convective structure changes inside the star. Before modelling this process, we test relations of growth rates and linear periods to the free parameters. This in...
Article
Full-text available
One of the first 'International Dark-sky Parks' in Europe was established at the Zselic Landscape Protection Area in Hungary. A special monitoring program has been carrying on to survey the quality of the night sky using 'Sky Quality Meters' and DSLR cameras. The main conclusion of our measurements is that the local villages have only a minimal eff...
Article
Full-text available
We present the first results of our analyses of selected RR Lyrae stars for which data have been obtained by the Kepler Mission. As expected, we find a significant fraction of the RRab stars to show the Blazhko effect, a still unexplained phenomenon that manifests itself as periodic amplitude and phase modulations of the light curve, on timescales...
Article
Light curve parameters and classification results are presented for all the objects measured in the first four CoRoT observing runs: IRa01, SRc01, LRc01, and LRa01. The information presented here should allow scientists to make candidate lists of their objects of study and to obtain some basic light curve information. We recall that we did not perf...
Article
Full-text available
Context. Aims. In this work, we describe the pipeline for the fast supervised classification of light curves observed by the CoRoT exoplanet CCDs. We present the classification results obtained for the first four measured fields, which represent a one-year in-orbit operation.Methods. The basis of the adopted supervised classification methodology ha...
Article
We present the preliminary results of our search for low amplitude single mode pulsations in objects located near the edges of the classical Cepheid Instability Strip (IS). Cepheids entering or leaving the IS are expected to show low amplitude pulsation, while stars outside the IS may pulsate in a high-overtone radial mode that is trapped in the st...
Article
Long period white dwarfs show changes in their frequency spectra from one observing season to another, i.e. their light-curves cannot be considered as stationary multiperiodic variations on long timescales. However, due to the complex frequency spectra of these stars and the narrow frequency spacing, it is still unknown, what the shortest time scal...
Article
Long-term observational data have information on the magnetic cycles of active stars and that of the Sun. The changes in the activity of our central star have basic effects on Earth, like variations in the global climate. Therefore understanding the nature of these variations is extremely important. The observed variations related to magnetic activ...
Article
Full-text available
We study the time variations of the cycles of 20 active stars based on decades-long photometric or spectroscopic observations. A method of time-frequency analysis, as discussed in a companion paper, is applied to the data. Fifteen stars definitely show multiple cycles; the records of the rest are too short to verify a timescale for a second cycle....
Article
One of the hardest questions of stellar pulsation theory is the modal selection in multi-mode oscillations. Linear stability analyses predicts a dense spectrum of excited modes; however, the amplitude of these modes cannot be determined by these calculations. Only nonlinear hydrodynamic models can answer the above question; however, codes exist onl...
Article
The analysis of the multicolour photometric observations of MW Lyr, a large modulation amplitude Blazhko variable, shows for the first time how the mean global physical parameters vary during the Blazhko cycle. About 1-2 percent changes in the mean radius, luminosity and surface effective temperature are detected. The mean radius and temperature ch...
Article
At the Zselic Landscape Protection Area an extensive program has been started to conserve the excellent night sky quality of the region. It is expected that an International Dark Sky Park will be established, following the recommendations of the International Dark Sky Association. We measured and modeled the artificial luminance of the night sky. T...
Article
Bigger and bigger parts of human societies realize that environment protection has to be handled seriously. Our environment cannot only be polluted by too much carbon dioxide emission or dropping litter but also by undesired light emission. In the study it is analyzed which photometric quantities are appropriate for describing the amount of light p...
Article
One-dimensional nonlinear pulsation codes are successful in modelling of many features of radial stellar oscillations. There are still many unsolved problems that require more realistic theoretical models. Three-dimensional calculations of full amplitude stellar models are impossible at the moment (and in the near future), so the radial codes still...
Article
Although public talks of astronomy and cosmology are usually attractive for the general audience, it is very hard to present specialized topics like stellar pulsation to the public at large. Fortunately parallels between the different disciplines, even between science and art, can help to popularize the interior physics of stars. For example, acous...
Article
Background in acoustics Variable stars show light variations due to internal acoustic waves. There are strong physical mathematical parallels between stellar behaviour and musical instruments: the basic principles underlying their “overtone” frequencies are identical. However, “stellar instruments” have many characteristics that make their sounds d...
Article
We present our new program package TiFrAn (Time Frequency Analyzer), a tool which was developed for analyzing variable star data. The main properties of TiFrAn: An extension to the Tcl scripting language: it makes it possible to write flexible scripts. Includes standard methods (DFT for unevenly sampled data, least square fits, etc). Time-frequen...
Article
We present the properties of a new software developed for examination of multi-periodic signals. The program package MuFrAn disposes a user-friendly graphical user interface and an extended Tcl/Tk script language suitable for definition of complex and repeatable tasks, and creating publication-quality Postscript figures. We have implemented the sta...
Article
Full-text available
We describe a methodology that allows us to follow the pulsational behavior of an RR Lyrae model consistently and automatically along its evolutionary track throughout the whole instability strip. It is based on the powerful amplitude equation formalism, and resorts to a judicious combination of numerical hydrodynamical simulations, the analytical...
Article
An analysis of the photometric observations of the light curves of the five large amplitude, irregularly pulsating stars R UMi, RS Cyg, V CVn, UX Dra and SX Her is presented. First, multi-periodicity is eliminated for these pulsations, i.e. they are not caused by the excitation of a small number of pulsation modes with constant amplitudes. Next, on...
Article
The pulsational properties of the Cepheid models along the evolutionary tracks from the Padova group (Girardi et al.), as calculated with our turbulent convective pulsation code, are in good agreement with the resonance constraints imposed by the observational OGLE-2 data of the Small and Large Magellanic Clouds. Our study suggests that the P_4/P_1...
Article
Asteroseismology is one of the major important science topics in astrophysics in the coming decade. Several recent breakthroughs have indeed been made in Europe in this field. The observational data will improve significantly in the near future, as upgraded ground-based instruments are being built and two European asteroseismic space missions are p...
Article
Full-text available
Everyone is fascinated with astronomy. Opportunities for astronomers to explain their work to the public with popular books, public talks, media interviews, television and radio programmes and internet sites are unlimited with very satisfying public response to all of our efforts. Recognising this, the directors of the European Network of Excellenc...